Injection unit for a pressure die casting machine

ABSTRACT

A pressing-in unit for a hot-chamber pressure diecasting machine permits an electro-mechanical drive for the casting plunger. In order to absorb the moments of inertia of the drive which occur during the switch-over of the casting plunger drive from the mold filling phase to the after-pressure phase, a spring element in the form of a plastic component or a liquid spring is inserted between the casting plunger and the pusher rod moved by the electric drive, which spring element is compressed by the additional path which the pusher rod still covers after the braking of the motor. The extent of the compression is selected such that the resulting axial force which is exercised by the plastic component or by the liquid spring is sufficient for causing the desired after-pressure in the molten metal.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a pressing-in unit for a pressure diecastingmachine, particularly for a hot-chamber pressure diecasting machine forprocessing molten metals, having a casting plunger for pressing thecasting material into a mold, which casting plunger can be acted upon byway of a pusher rod which is connected with a linear drive driven by anelectric motor, which linear drive, after the filling phase of the mold,is held in a pressure phase for achieving a pressure in the castingmaterial, a spring element being connected between the casting plungerand the pusher rod.

From European Patent Document EP 0 430 616 A1, a pressing-in unit isknown, in which a spindle is provided as a linear drive for driving anut. The drive of the casting plunger connected with the linear drivetakes place by way of a belt driven by an electric motor. In this case,the belt drive acts upon the spindle by way of an electromagneticallycontrollable disk coupling so that, in this manner, the advancing speedof the casting plunger and, after the filing of the mold, the pressureto be maintained in the casting material can be controlled. Arotational-speed-dependent signal, which is emitted by a speedometerconnected with the spindle drive, is used for the control. A drive ofthis type for the pressing-in unit requires relatively highexpenditures. Mainly because of the susceptibility of such couplings towear, the controlling and regulating of the disk coupling hasdisadvantages.

From the Patent Abstract of Japanese Patent 0 7155 925, a pressing-inunit of the initially mentioned type for a pressure diecasting machineis known, in which an elastic spring element is connected between thecasting plunger and the pusher rod and avoids undesirable pressure peakswhich, as a result of the system, arise during the stopping of the driveand during the transition to the regulating of the pressure because ofthe forces of inertia inherent to the drive. While such problems do notoccur in the case of plastic injection molding machines because of theelastic behavior of liquid plastic masses, the conditions in the case ofpressure casting machines for processing molten metal are different inthat the molten metal can virtually not be compressed.

In the case of injection molding machines for the processing of liquidplastic masses, it is also known to control the extruder spindlesprovided there for the pressing-out by means of electronic regulatorssuch that, in the critical phases of the ram pressure buildup during themelting as well as when maintaining the pressure in the after-pressurephase, the desired advancing rates or torques can be exercised formaintaining the pressure (German Patent Document DE 43 45 034 A1).Information is also supplied there that a similar driving principle canalso be used in the case of pressure casting machines if an elasticelement is connected between the drive and the movable injectionelements.

It is an object of the present invention to construct a pressing-in unitof the initially mentioned type in a manner which is as simple aspossible.

In order to obtain a simple type of construction, it is provided in thecase of a pressing-in unit of the initially mentioned type according tothe invention that the spring element is constructed as an elasticplastic component or as a liquid spring and is designed such that thereaction force onto the casting plunger resulting from its prestressingwill be high enough for applying the axial force to the casting plungerwhich is required for achieving the required pressure in the castingmaterial. Such a spring element can absorb the moment of inertiaoccurring during the braking of the drive. The resulting spring travelprevents the further displacement of the casting plunger and thus alsothe occurrence of pressure peaks without the requirement of complicatedcontrol measures.

Thus, with respect to a pressing-in unit of the initially mentionedtype, two objects are achieved according to the invention. On the onehand, the moments of inertia caused during the braking of the drive bythe masses of the electric motor and of the transmission can beabsorbed. Thus, as of the point in time of the switch-over from fillingthe mold to pressure, the casting plunger is no longer advanced. On theother hand, the additional travel occurring as the result of the momentsof inertia is used for prestressing the spring element and the thengenerated prestress is utilized for acting upon the casting plunger bymeans of an axial force which is high enough for maintaining the desiredafter-pressure in the casting material.

These measures simplify the controlling of the casting plunger. Thereason is that the regulating of the after-pressure can be reduced inthis manner to a secondary speed control in a regulator cascade with aprimary automatic power control.

As a further development of the invention, a sensor, which is connectedwith the control of the drive, can be assigned to the spring element.This sensor detects the axial force exercised by the spring element andcorrespondingly influences the drive control. As a result, the electricdrive can be controllable such that the spring element is not compressedbeyond a predefined extent.

As a further development of the invention, an elastic plastic ring canbe provided as a spring element which is held between two flanges, ofwhich one is held on the pusher rod and the other is fixedly connectedwith a sleeve telescopically guided on the pusher rod. The relativemovement between the pusher rod and the sleeve compresses the plasticring, specifically by an extent which corresponds to the adjusting pathof the pusher rod caused by the after-running of the drive after thebraking operation. It was found that such a plastic ring, particularlyif it consists of a coated polyurethane caoutchouc, can apply the highforces occurring in the case of pressure diecasting machines for thecompression of the molten metal. This plastic ring can also beconstructed of two or several parts which can be deformed successively,so that different spring characteristics can also be implemented for theabsorption of the moment of inertia and then for the application of theaxial pressure force.

In a constructively simple manner, the pusher rod may have an extensionof a smaller diameter which penetrates the plastic ring and on which thesleeve is also carried. The step provided between the extension and thepusher rod can then be used as a bearing for the disk held on the pusherrod and for a pressure sensor assigned to this disk. It is alsoconceivable in this case that the spring element and the arrangement ofthe sleeve are coordinated with one another such that the extension canbe adjusted only a certain path relative to the sleeve.

As in the prior art, a threaded drive can be provided in a simple manneras the linear drive which consists of a spindle and a nut guidedthereon. However, it is also conceivable to provide a rack-and-piniondrive as the linear drive which permits a robust construction and alsohas a low noise development. The rack-and-pinion drive can take place byway of a transmission which is applied to both sides of the rack, sothat no one-sided stressing of the rack occurs. In the case of such amethod of construction, two motors can also be provided for driving thetransmission so that a power adaptation with higher dynamics as well asa higher acceleration and deceleration becomes conceivable. Thetransmission may be completely encapsulated, so that no oil outflow ispossible in the direction of the molten bath.

However, the linear drive can finally also be a carriage guide driven bya connecting rod of a crank mechanism, in which case, if a certainvertical adjustment is provided, an optimal adaptation of the powercourse is conceivable to the mold filling stroke.

However, a liquid spring, which is known per se, can also be provided asthe spring element (“LUEGER Lexikon der Technik”, Volume 12—AutomotiveEngineering—Edition 1967, Verlag DVA Stuttgart, Page 223). At thepressing-in forces in the order of several tons occurring in the case ofpressure diecasting machines, such cylinder/piston units, which as arule are filled with oil, can provide the required spring travel, inwhich case, the spring force can then also here be utilized for actingupon the casting plunger.

In a practical embodiment, the liquid spring can be provided with animmersion piston which, by the force of a spring, is pressed into theinterior of the cylinder in order to subject the liquid to a certainprestress even before it is acted upon by the pusher rod. As a furtherdevelopment of the invention, the cylinder of the liquid spring can alsobe provided with an opening to which an excess pressure limiting valveand a pump are connected for the possible return of exited liquid. Acompressive strain converter can be connected to the connection line tothe pressure limiting valve, which converter can, in turn, be connectedwith the control of an electric-motor drive.

However, additional liquid volumes can advantageously also be connectedto the connecting line. The controllable connection of the liquid volumecontributes to the change of the spring characteristic. Also when aliquid spring is used, an adaptation of the spring characteristic canalso be achieved in this manner which is similar to that of a multipartplastic ring described above.

The invention is illustrated in the drawing by means of embodiments andwill be explained in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a pressing-in unit for a hot-chamber pressurediecasting machine for processing molten metal;

FIG. 2 is a view of a pressing-in unit similar to FIG. 1 but with arack-and-pinion drive instead of a threaded spindle drive;

FIG. 3 is a schematic representation of a sectional view of the unit ofFIG. 2 in the direction of the intersection line III—III;

FIG. 4 is a representation similar to FIG. 3 but with anothertransmission arrangement for acting upon the rack;

FIG. 5 is a view of a pressing-in unit for a hot-chamber pressurediecasting machine similar to FIG. 1, but with a crank drive for thepusher rods;

FIG. 6 is a schematic representation of the plastic spring used in FIGS.1, 2 and 5;

FIG. 7 is a view of a modified construction of the plastic springaccording to FIG. 6;

FIG. 8 is a view of the course of the force path caused by the plasticspring according to FIG. 6;

FIG. 9 is a view of the course of the force path when using a plasticspring according to FIG. 7; and

FIG. 10 is a view of an embodiment of a liquid spring which can be usedinstead of the plastic spring according to FIG. 6 or 7 between thepusher rod and the casting plunger of the pressing-in units.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, an electric motor 1, for example, an asynchronous motor orother variants of servo motors, is provided which has a transmission,which is not shown in detail, and a coupling part 2 which drives athreaded spindle 3 to carry out a rotating movement. The threadedspindle 3 is guided in a protective housing 5 in a sealed manner. A nut4 is guided on this spindle 3 and interacts with the thread of thespindle 3. This nut 4 engages by means of a guiding cam 6 in a groove 7inside the housing 5 and is therefore non-rotatably guided in thehousing 5. By way of an extension 8 reaching over the free end of thespindle 3, the nut 4 is connected with a pusher rod 9 which, in turn,extends in a sealed-off manner out of the housing 5 and is provided withan extension 10 of a smaller diameter. On the extension 10, a first disk11 is movably guided which rests against a pressure sensor 12 which maybe constructed, for example, in the manner of a piezoelectric element.By way of a signal line 13, this pressure sensor 12 is connected with amultiparameter regulator which is not shown because it is known andwhich regulates the motor 1.

Furthermore, a sleeve 14 with an end disk 15 is displaceably disposed onthe extension 10, in which case a spring element in the form of aplastic ring 16 is arranged between the end disk 15 and the disk 11resting against the pressure sensor 12. This plastic ring 16 is alsopenetrated by the extension 10. At the end facing away from the disk 15,the sleeve 14 is provided with a connection end 17 for the connectionwith the casting plunger which is not shown, the free end of theextension 10 being provided with a step 18 of a larger diameter whichholds the sleeve on the extension 10 and can also be used for a certainprestressing of the plastic ring 16. This step 18 is away from an innerend surface 18 of the sleeve 14 by the distance a. The operation of thispressing-in unit will now be started for pressing molten metal in aknown manner from the crucible of a hot-chamber pressure diecastingmachine through the casting cylinder and an ascending pipe into themold. Then the electric drive 1 is activated by way of themultiparameter regulator, which is not shown, for rotating the spindle3, which has the result that the nut 4 moves from its illustratedposition along the spindle 3 downward and in the process also pressesthe pusher rod 9 downward, specifically at the speed required for thefilling operation of the casting mold.

When the mold is filled, the rotary drive of the spindle 3 must beswitched from speed control to torque control and the motor 1 is brakedfor this purpose. Since the motor as well as the transmission, which isnot shown in detail, and all parts caused to rotate by the drive have amass-caused moment of inertia, it is not possible to immediately,starting from the point of time of the switch-over, prevent thecontinued rotation of the spindle 3. In order to avoid in this case thatthe casting plunger, which is not shown, continues to press onto theincompressible molten metal situated in the mold and undesirablepressure peaks are caused thereby in the molten metal or force peaksoccur in the driving mechanism, which may result in damage, the springelement 16 is provided which compresses in this case and absorbs thepath which otherwise the casting plunger would additionally have tocover. Because of the incompressibility of the molten metal in the mold,the casting plunger will virtually stand still after the filling of themold, while the pusher rod 9 and its extension 10 continue to be moved acertain distance which can, however, be absorbed by the relativedisplaceability between, the sleeve 14 and the extension 10 by thespring element 16.

In this case, the arrangement is such that the distance still covered bythe drive is shorter than the measurement a. The spring element 16 istherefore compressed by an amount slightly less than a and is thereforetensioned. In this case, the design can be such that, when the springtravel is adjusted—which is therefore smaller than a—a reaction forceexercised by the spring element 16 acts upon the sleeve 14 and thus onthe casting plunger and is sufficiently large for causing in the moltenmetal the required after-pressure as the result of a force, for example,in the order of from 7-8 tons (70-80 KN) The plastic ring used in theembodiment can apply these forces without being large in size. The useof a liquid spring would also be conceivable, in the case of which thecompressibility of liquids, particularly of oil at high pressures, isutilized. The motor 1 can be controlled to a holding torque, so that nocomplicated measures are required for maintaining the after-pressure inthe molten metal.

In order to, in each case, keep the force exercised by the springelement 16 so large that the required pressure is generated in themolten metal, the pressure sensor 12 is provided which emits ameasurement for the force exercised by the spring 16 upon the sleeve 14and thus on the casting plunger. As explained above, the disk 11 isdisplaceably arranged on the extension 10. The disks 11 and 15 aretherefore acted upon by the same axial force. This force can becontrolled to a very specific intensity by the above-mentionedmultiparameter regulator and the driving motor 1 as long as it isprovided that the deflection path for the spring element 16 is no largerthan the measurement a.

FIG. 2 is a schematic representation of a pressing-in unit similar toFIG. 1. The same reference numbers were therefore retained for identicalparts.

The difference is that a toothed rack 20 is fixedly connected with thepusher rod 9. The spindle drive according to FIG. 1 is therefore notpresent. Like the spindle 3 of FIG. 1, the toothed rack 20 is guided ina sealed-off manner in the housing 5 and in the toothed-rack housing 25which is closely connected with this housing 5. Transmission oil cantherefore not leak out and possibly enter into the molten metal.

As illustrated in FIG. 3, the toothed rack 20 is provided with atoothing on both sides and, by means of these toothings rests in eachcase on a pinion 23 and 24 which is disposed on one shaft 21 and 22respectively. In this embodiment, the shaft 21 is driven, specificallyby way of two motors 25 and 27 which are each in a driving connectionwith the shaft 21 by way of a coupling 28. A power adaptation can ineach case take place by controlling the couplings 28 in so far as thedrive of the toothed rack takes place only by way of one of the electricmotors or by way of both electric motors 26 and 27. As a result of thisfurther development, higher dynamics and a higher acceleration anddeceleration can be exercised on the toothed rack 20. As a result, theadjusting speeds for the toothed rack and the pusher rod 9 can bemaintained to be relatively high.

FIG. 4 shows a toothed rack arrangement similar to that of FIG. 3 onlythat here the toothed rack 20′ is driven by pinions 29 and 30 which, inturn, are part of a transmission consisting of additional pinions 31,32, 33 and of the gears 34 and 35, in the case of which the gear 35,which is fixedly disposed on the shaft 36, is again optionally driven byone or both electric motors 26, 27. As a result of such a transmissionarrangement, transmission ratios can be realized which ensure aparticularly good functioning of the pressing-in unit.

FIG. 5, in which reference numbers which refer to the same parts asthose used in FIG. 1 were also retained, illustrates a deviation fromthe construction such that the pusher rods 9 are acted upon by acarriage 37 which is guided in a straight-line mechanism 38 in thedirection of the axis of the pusher rods 9. This carriage 37 is actedupon by a connecting rod 39 which is actuated by a crank disk which, inturn, can be moved by way of a transmission not shown in detail by oneor—or two—electric motors 41.

FIG. 6 shows the plastic ring 16, as used in all illustrated embodimentsof FIG. 1, 2 or 5 as a spring element between the pusher rod 9 and thecasting plunger. This plastic ring 16 consists of a coated polyurethanecaoutchouc. It can then, when it is compressed in the direction of thearrow s, generate the force path characteristic illustrated in FIG. 8.In FIG. 8, F indicates the force and s indicates the deformation path ofthe plastic ring 16.

FIG. 7 shows an embodiment in which the plastic ring 16 is replaced bytwo mutually concentrically arranged ring parts 16 a and 16 b, which canalso consist of coated polyurethane caoutchouc and, according to FIG. 9,can generate a different path/force characteristic. When the plasticring 16 a is compressed by the amount S1, the force F exercised by itincreased to a defined amount which is illustrated schematically in FIG.9 when reaching the distance s1. However, when the two plastic rings 16a and 16 b are further compressed beyond the distance s1 along thedistance s2, they act jointly as a considerably stronger spring, so thatthe rise in force per distance unit becomes significantly larger. Thisfurther development can be utilized for increasing the pressure forceexercised on the molten metal, without the requirement ofhigh-expenditure control measures for the drive.

FIG. 10 finally shows a liquid spring 42 which can be connected, insteadof a plastic spring element, also between the pusher rod and the castingplunger. This liquid spring 42 consists of a stable cylinder 43 which isprovided with a connection bore 44 for mounting a part provided with theconnection part 17 (FIG. 1), which part, in turn, receives the castingplunger which is not shown. A piston rod 46, which is guided in a sealedmanner in the lid 45, projects into the interior of the cylinder 43which is closed by the lid 45 and which is filled by means of a volumeV₁ of a pressure oil. At its lower end, this piston rod is provided witha plate-shaped attachment 47 which is acted upon by a pressure spring 48supporting itself on the lid 45. This pressure spring 48 thereforepresses the piston rod 46 by a defined amount into the volume V₁ sothat, as a result, the pressure in the liquid contained in the cylinder43 is increased by the piston rod 46 acting as a displacement body.

As indicated at the beginning, in the case of this liquid springarrangement, the compressibility of liquids at very high pressures isutilized. Just as the above-mentioned plastic spring bodies, liquidsprings are therefore suitable for absorbing the high forces occurringin the case of pressure diecasting machines.

In the illustrated embodiment, the cylinder 43 has an opening 49 whichis connected by way of a connection line 50 with a pressure limitingvalve 51. Liquid possibly flowing out by way of the pressure limitingvalve 51 is collected in a schematically indicated container 52 and isreturned by way of a pump 53 to the interior of the cylinder 43. Apressure strain converter 54, which may, in turn, be connected with thecontrol of the electric driving motors, is connected to the connectionline 50. In this manner, the desired pressing-in force onto the moltenmetal can be perfectly adjusted and maintained by way of the drive.

However, another line 55 which, by way of a control valve 56, permitsthe connection of an additional volume V₂ or—by way of the connectionline 57—also the connection of additional volumes, which areschematically marked V_(x), is also connected with the connection line50. This further development allows a changing of the force pathcharacteristics exercised by the liquid spring 52 in a similar mannerand its adaptation to the respective requirements, as was explained bymeans for FIG. 9 for the two-part plastic ring.

As a result of the further development according to the invention, afirm relationship therefore exists between the deflection path (which ispreferably smaller than a—FIG. 1—of the spring element 16 or 42 and theaxial force exercised on the casting plunger. The invention thereforepermits the drive of a pressing-in unit for hot-chamber pressurediecasting machines by electro-mechanical means and offers the advantageof a relatively simple controlling and regulating.

What is claimed is:
 1. Pressing-in unit for a hot-chamber pressurediecasting machine for processing molten metals, having a castingplunger for pressing casting material into a mold, which said castingplunger can be acted upon by way of a pusher rod which is connected witha linear drive driven by an electric motor, said linear drive, after afilling phase of the mold, being held in a pressure phase for achievinga necessary pressure in the casting material, a spring element beingconnected between the casting plunger and the pusher rod, wherein thespring element comprises one of an elastic plastic component and aliquid spring and provides a reaction force onto the casting plungercaused by stress of the spring element large enough for applying anaxial force required for achieving the necessary pressure in the castingmaterial during the pressure phase.
 2. Pressing-in unit according toclaim 1, wherein a sensor, which is connected with the control of thedrive and detects the axial force exercised by the spring element, isassigned to the control of the drive.
 3. Pressing-in unit according toclaim 1, wherein the electric linear drive can be controlled such thatthe spring element is not compressed beyond a defined extent. 4.Pressing-in unit according to claim 1, wherein the spring element is anelastic plastic ring which is held between two disks of which one isheld on the pusher rod and the other is fixedly connected with a sleevetelescopically guided on the pusher rod.
 5. Pressing-in unit accordingto claim 4, wherein the plastic ring is constructed of two or more partswhich can be successively deformed.
 6. Pressing-in unit according toclaim 1, wherein the spring element comprises a plastic component (16,16 a, 16 b) which consists of a coated polyurethane caoutchouc. 7.Pressing-in unit according to claim 4, wherein the pusher rod has anextension of a smaller diameter which penetrates the spring element,formed as a plastic ring, and on which the sleeve is guided. 8.Pressing-in unit according to claim 4, wherein the spring element andthe arrangement of the sleeve are mutually coordinated such that anextension can only be adjusted a certain distance relative to thesleeve.
 9. Pressing-in unit according to claim 2, wherein the sensorrests on a step formed between the pusher rod and an extension of asmaller diameter and on a disk which is guided on an attachment andadjoins the spring element.
 10. Pressing-in unit according to claim 1,wherein a threaded drive is provided as the linear drive which comprisesa spindle and a nut guided thereon.
 11. Pressing-in unit according toclaim 1, wherein a rack-and-pinion drive is provided as the lineardrive.
 12. Pressing-in unit according to claim 11, wherein the drive ofa toothed rack of the rack and pinion drive takes place by way of atransmission which is applied to both sides of the toothed rack. 13.Pressing-in unit according to claim 12, wherein two motors areoptionally provided for driving the transmission.
 14. Pressing-in unitaccording to claim 1, wherein the linear drive is a carriage which isdriven by a connecting rod of a crank drive and is held in astraight-line mechanism.
 15. Pressing-in unit according to claim 1,wherein the spring element is a liquid spring which comprises a cylinderwith an immersion plunger rod, the immersion plunger rod being pressedby force of a spring into an interior of the cylinder in order tosubject a liquid to a defined prestress even before being acted upon bythe pusher rod.
 16. Pressing-in unit according to claim 15, wherein thecylinder of the liquid spring is provided with an opening to which anexcess pressure limiting valve and a pump are connected for possiblereturning of leaked-out liquid.
 17. Pressing-in unit according to claim16, wherein a pressure strain converter is connected to a connectionline to the pressure limiting valve and is connected with the control ofthe electric driving motor.
 18. Pressing-in unit according to claim 16,wherein additional liquid volumes are connected to the connection line,the controllable connection of said volumes contributing to changing thespring characteristic.
 19. Pressing-in unit according to claim 2,wherein the electric linear drive can be controlled such that the springelement is not compressed beyond a defined extent.
 20. Pressing-in unitaccording to claim 19, wherein the spring element is an elastic plasticring which is held between two disks of which one is held on the pusherrod and the other is fixedly connected with a sleeve telescopicallyguided on the pusher rod.
 21. Pressing-in unit according to claim 4,wherein the spring element comprises a plastic component which consistsof a coated polyurethane caoutchouc.
 22. Pressing-in unit according toclaim 1, wherein the spring element is a liquid spring which comprises acylinder with an immersion plunger rod, the immersion plunger rod beingpressed by the force of a spring into the interior of the cylinder inorder to subject the liquid to a defined prestress even before beingacted upon by the pusher rod.
 23. Pressing-in unit for a hot-chamberpressure diecasting machine for processing molten metals, which has acasting plunger for pressing casting material into a mold, which saidcasting plunger can be acted upon by way of a pusher rod which isconnected with a linear drive, which said linear drive, after a fillingphase of the mold, is held in a pressure phase for achieving a necessarypressure in the casting material, a spring element being connectedbetween the casting plunger and the pusher rod, wherein the springelement is configured such that reaction forces onto the casting plungercaused by the spring element are large enough for applying an axialforce required for achieving the necessary pressure in the castingmaterial, wherein the spring element is an elastic plastic ring which isheld between two disks of which one is held on the pusher rod and theother is fixedly connected with a sleeve telescopically guided on thepusher rod.
 24. Pressing-in unit according to claim 23, wherein asensor, which is connected with the control of the drive and detects theaxial force exercised by the spring element, is assigned to the controlof the drive.
 25. Pressing unit according to claim 23, wherein controlmeans are provided for controlling the linear drive such that the springelement is not compressed beyond a defined extent.
 26. Pressing-in unitaccording to claim 23, wherein a threaded drive is provided as thelinear drive which comprises a spindle and a nut guided thereon. 27.Pressing-in unit according to claim 23, wherein a rack-and-pinion driveis provided as the linear drive.
 28. Pressing-in unit according to claim23, wherein the linear drive is a carriage which is driven by aconnecting rod of a crank drive and is held in a straight-linemechanism.